1. Field of the Invention
The invention relates to presses for hydrostatic extrusion with a die support and particularly to sealing such presses at the die end.
In presses for hydrostatic extrusion very high pressure levels are used; usually the working pressure is between 10 and 20 kbar. At this high pressure a reliable seal must be achieved between the inner wall of a press cylinder and a die or a die support and between the inner wall of the cylinder and a punch projecting into the cylinder, said punch or piston, when being inserted into the cylinder, producing the high extrusion pressure in a pressure medium which is enclosed in a pressure chamber formed of the cylinder, the die and the piston. A billet to be extruded is located in the pressure chamber and is pressed out through the die under the effect of a pressure medium. On raising the pressure from atmospheric level to the extrusion level, the inner diameter of the cylinder increases by 0.5 to 1%, which requires a seal to prevent pressure medium from flowing out through a gap between the cylindrical wall and the piston and the die, respectively, which expands upon a pressure increase. Any leakage at the high pressures mentioned may involve severe damage to the die or the die support, which requires an exchange of these expensive parts.
2. The Prior Art
In U.S. Pat. No. 3,702,555 there is shown and described in more detail a press of the kind referred to in this invention. Larker application Ser. No. 462,402, filed Apr. 19, 1974 and U.S. Pat. Nos. 3,865,387 and 3,877,707 show different variations of high pressure seals for presses for hydrostatic extrusion, having a first sealing ring abutting the cylindrical wall and a seal holder and a second sealing ring sealing against a die or piston and a seal holder. These seals have provided great improvement and have resulted in a great increase in the service life. However, certain drawbacks have remained.
In some cases leakage has occurred between the metallic sealing rings hitherto used and a cooperating sealing surface on a seal holder. Leaking pressure medium causes surface damage, which means that sealing will not be obtained during a subsequent pressing. O-rings providing initial sealing upon a pressure increase in the pressure chamber will improve the seal. At the high pressures used, rubber and plastic rings will become brittle and will easily break, and therefore a good, pure metallic seal is always advantageous. In hot extrusion the seal is also heated by the billet which has been heated to 600.degree. C., which increases the strain further. In addition, the viscosity of the pressure medium is reduced, which also increases the problems of sealing.